Micro-Electro-Mechanics Systems (MEMS) are fabricated from metals such as aluminum, tungsten, nickel, etc. In addition MEMS are fabricated from polycrystal materials such as polysilicon and single crystal (SC) materials such as Single Crystal Silicon (SCS).
MEMS respond to their physical environment or act thereupon. It is important to be able to sense the behavior of MEMS in response to the environment. Sensors used conventionally to sense behavior of general electromechanical systems are capacitive, piezoresistive, piezoelectric, or optical. Other devices are known that use a floating gate in MOS configurations as a sensor. The sensing mechanisms used in the art may be classified into two broad categories: displacement sensors and stress sensors, i.e., sensors that sense the stress induced by the displacement. The capacitive, floating gate MOS, and optical sensors belong to the family of devices that sense the displacement directly. The piezoresistive and piezoelectric sensors belong to the family of stress sensors.
The sensing mechanism is not necessarily part of the device being sensed. Some examples are capacitance sensors, optical sensors and tunneling sensors. In capacitance sensing, the capacitance between a moving electrode and a stationary electrode is used as a sensing method. The electrodes are conductive or are coated with a conducting material. In optical sensing, the deflection of a structure is used to deflect light. Tunneling sensors use the tunneling current to sense the deflection of the moving part. The tunneling current and the light used in optical sensors are external to the mechanical phenomena, i.e., they are external to the deflection of the structure.
Piezoresistive sensors, on the other hand, are part of the device being sensed. Here the moving structure is designed so that its movement is translated into an electrical signal that is induced by the piezoresistive change of the structure itself.
A disadvantage of the displacement sensors is that in many cases (that is, capacitive, floating gate MOS) it is necessary to have a fixed structure in a close vicinity of the moving part. In some cases this fixed structure interferes with the measurement itself For example, in microphones the back electrode is the fixed structure, and the capacitance between this electrode and the membrane is used as a sensor. In general, the back electrode is too close to the membrane and thus damps the response of the microphone. In order to by-pass this problem, it is necessary to create perforations in the back electrode. This increases the complexity and cost of the device. Moreover, the abovementioned problems make these sensors impractical for MEMS.
In general sensing devices have small outputs. Therefore, it is necessary to feed the output signals into a preamplifier before they are further amplified. This adds an additional complexity and cost to the device, especially for MEMS.
The integration of transistors into devices for sensing purposes has been suggested. The electrical properties of a pn-junction in a tunnel diode were used to sense the response of microphones, as discussed in E. S. Rogers, "Experimental tunnel-diode electromechanical transducer elements and their use in tunnel-diode microphones," J. Acoust. Soc. Am., 34, 883-(1962). The electrical properties of transistors were used to sense the response of accelerometers, as discussed in M. E. Sikorski, P. Andreatch, A. Grieco, H. Christensen, "Transistor microphone", Rev. Sci. Instrum., 33, 1130-, (1962) and B. Puers, L. Reynaert, W. Snoeys, W. M. C. Sansen, "A new uniaxial accelerometer in silicon based on the piezojunction effect", IEEE Trans. Electron Device, ED-35, 764-, (1988).
In addition, a `floating gate` in an MOS configuration was used as a sensing mechanism for a condenser microphone, as discussed in K. Kuehnel, "Silicon condenser microphone with integrated field effect transistor", Sensors and Actuators A, 25-27, 521-525, (1991).
However, the prior art does not discuss or teach integration of transistors with beams.